1. Field of the Invention
The present invention relates to image fixing employed by electrophotographic image formation apparatuses such as a copying machine, a printer, and a facsimile apparatus, and especially relates to a fixing method, a fixing apparatus, an image formation method, and an image formation apparatus that realize energy savings of thermal fixing a toner image to a recording medium.
2. Description of the Related Art
In recent years and continuing, resource and energy conservation are increasingly required for environmental protection of the earth. In electrophotography, R&D are advancing in order to reduce power consumption for energy savings, especially in the field of thermal fixing technology that requires intensive power consumption. At present, fixing temperatures ranging about 150° C. to 200° C. are commonly used, and starting time from the room temperature of a fixing apparatus takes about 1 to 5 minutes. It is desired that the temperature be reduced to 150° C. or less, preferably about 100° C. for energy savings, and the starting time be shortened.
There are various approaches to realize energy savings. One of the approaches is reducing the power consumption when transporting a recording medium. In order to realize this, an important point is lowering the melting point of toner. Namely, the softening point or the melting point of the toner should be reduced to 100° C. or less in order to realize fixing at a lower temperature. However, as for a given kind of organic high polymer, if the melting point is reduced, the fusion viscosity of the toner will be reduced. The reason for this is considered to be because the melting point of resin is dependent on molecular weight, tangles of molecule chains become loose if molecular weight is low, interaction becomes weak, and the fusion viscosity falls. Accordingly, when the melting point of the toner is low, the viscosity rapidly decreases at a temperature higher than the melting point. For this reason, the effective rubber margin (temperature region of the toner showing characteristics of rubber) becomes narrow and the toner on the recording medium tends to be offset toward a heating object for fixing. Then, it is conceivable that the contact time of a fixing surface contacting the toner be lengthened so that heat may fully get across to the toner at the time of fixing. In order to realize this with an apparatus with a usual roller fixing method, a technology is known whereby the diameters of a fixing roller and a pressurization roller (the rollers) are increased such that the time of the toner contacting the fixing surface can be lengthened by deformation of a rubber surface of the pressurization roller at the time of pressurization.
However, since the diameters of the rollers are enlarged, the size of the fixing apparatus becomes large. In order to solve this, a belt is wound around the heating roller such that adhesion time is increased. However, the problem with this approach is that the starting time is increased due to the mass of the rollers and the mass of an auxiliary roller, i.e., the starting time becomes long.
In general, thermal fixing is carried out by bonding a toner image imprinted on the recording medium, such as paper, by heating and pressurizing such that the toner is fused and bonded to the recording medium that is sandwiched by the heating roller and the pressurization roller. In this way, heat is transferred from the heating roller to the toner and the recording medium. However, since there is a limit in the width of the nip constituted by the heating roller and the pressurization roller, heating time is sharply limited. The higher the temperature of the heating roller is, i.e., the greater the heat slope (difference between the temperature of the heating roller and the environmental temperature) is, the quicker a greater quantity of heat flows from the heating roller to the toner and the recording medium.
However, if the temperature is high and a great amount of heat is transferred to the recording medium, energy consumption inevitably increases. This is not desirable from the viewpoint of energy savings. Although the nip width constituted by the heating roller and the pressurization roller may be enlarged by increasing the amount of deformation of the rubber and sponge of the pressurization roller by lowering the hardness such that a longer heating time be obtained in the case of the fixing apparatus using the rollers, there is a limit in extending the heating time due to the curvature of the rollers. That is, if the radius of curvature of the rollers is increased, a greater nip width becomes available; however, the heating roller of the increased diameter requires an excessively great heat capacity, and energy consumption is increased by the increase of heat dissipation area. Accordingly, the time for the temperature to rise increases.
With these limitations, roller fixing is often carried out by raising the temperature of the heating roller to a temperature beyond necessity in order to raise the temperature of the toner to a predetermined temperature within a short time corresponding to the limited nip width. In this case, since the temperature of the toner has to be raised to the melting point of the toner, the temperature of the heating roller is made high. The high temperature raises the surface temperature of the recording medium, such as paper. Further, since the heat slope is great, the temperature inside the recording medium is raised beyond necessity.
That is, in the case of roller fixing wherein only a small nip width is available, the temperature of the heating roller has to be raised beyond necessity, there is excessive heat dissipation due to the great heat slope of the roller as compared with the environment, and there is excessive transfer of the heat from the heating roller to the recording medium due to the high temperature of the heating roller. For this reason, the problem is in that excessive energy consumption occurs and the time for the temperature rise of the roller is long.
In order to solve the problem, it is conceivable that a fixing belt be used. In this case, the heat of the fused toner is dissipated for solidifying, and then the recording medium is separated from the fixing belt, thereby preventing the offset of the toner image from occurring. That is, the recording medium is heated through the fixing belt from the heating object for a long time. This method requires a low temperature compared with the roller fixing method; however, this method requires that the recording medium be heated for a long time such that the toner reaches the predetermined temperature. It takes a long time because the heating object provides a low temperature compared with the roller fixing method, producing a small heat slope, and taking a long time for the toner on the recording medium to reach the predetermined temperature.
According to this method, it is not necessary to raise the temperature beyond necessity and energy savings can be attained as compared with the roller fixing method that requires a temperature higher than the temperature at which the toner can be fixed. That is, since the recording medium is continuously heated at a low temperature while passing the heating section, although it takes a long time for the toner to reach the predetermined temperature, the toner on the recording medium can reach the same temperature as in the case of roller fixing. Further, since the temperature of the heating object at this time is low and the heat slope between the environment and the heating object is small, the heat dissipation becomes small, and since the temperature is low, the heat slope between the recording medium and the heating object is also small, and the total heat that the recording medium receives also becomes small.
While, indeed, fixing is carried out at a low temperature according to this method, another problem arises. That is, this method requires additional drive rollers, etc., increasing the mass of members that are to be heated, which in turn makes the temperature rising time of the fixing apparatus longer than the roller fixing method.
Further, since the toner image that has been solidified with heat having been dissipated is stuck to the surface of the belt, the recording medium tends to roll (curl) around the belt at a discharge section. In order to prevent the recording medium from rolling (curling), a separation nail, and the like, is often provided such that the separation nail hooks a tip of the recording medium for separating from the belt. When there is an image at the tip of the recording medium, there is a problem in that a scratch is produced.
According to Patent Reference 1, a cooling fan that is connected to a duct is provided for supplying fresh air to the surface of the belt that is wound around the fixing roller such that the toner in a fusion state is cooled and solidified on the recording medium and the offset is prevented from occurring. In the case of this technology, problems are that the recording medium tends to float while being conveyed, and the image tends to blur.
A system according to Patent Reference 2 includes a fixing belt that is wound around two or more rollers, and a heating unit, wherein guide rollers are prepared at the forward tip, rear tip and in-between of the conveyance passage of the recording paper so that the recording medium is conveyed being pressed down with its image side contacting the fixing belt. In this case, the problem is in that the recording medium tends to float at a portion where there are no guide rollers, and the image becomes blurred.
According to Patent Reference 3, an endless fixing belt is wound around a heating roller, a fixing roller, and a pressurization roller that contacts and pressurizes the fixing belt from underneath, wherein the fixing roller and the heating roller contact the pressurization roller constituting a nip between the rollers. In this case, image blur due to transporting the recording paper through a high curvature of the fixing nip is a problem.
Another approach to save energy is shortening the temperature rising time, the time being required until a fixing apparatus becomes ready to operate. In this approach, a user waiting time serves as an important element. That is, if the user waiting time should be short, the temperature of the fixing roller has to be maintained at a given temperature (standby temperature), which requires continuous and considerable power consumption while the fixing apparatus is not in use. Accordingly, if the temperature rising time can be made short without continuously supplying the power, it results in energy savings. However, as described above, the increase in the time of the toner contacting the fixing surface has a result opposite to shortening the temperature rising time until the fixing apparatus becomes usable.
According to a publicly known technology, a recording medium is supported by a pair of belts, and the recording medium is separated after the toner is cooled and solidified. In this way, hot offset is prevented from occurring (for example, Patent Reference 4). However, heat should be fully transferred for fixing, and if the toner is cooled, its performance is degraded. Furthermore, cooling the belts and the recording medium means throwing energy away, which is counter to energy savings.
In addition, the conventional technologies include a fixing apparatus and a fixing method wherein the temperature is maintained by sandwiching the recording medium between a pair of belts (for example, Patent Reference 5), an apparatus wherein the tension of a belt is considered (for example, Patent Reference 6), an apparatus wherein the time for the recording medium to pass a nip is considered (for example, Patent Reference 7), an apparatus wherein pressure within a nip constituted by a heating roller and a pressurization roller is considered (for example, Patent Reference 8), and an apparatus wherein diameters of a heating roller and a pressurization roller are considered (for example, Patent-Reference 9).                [Patent reference 1] JPA 5-019646        [Patent reference 2] JPA 10-221982        [Patent reference 3] JPA 2000-89593        [Patent reference 4] JP 51-29825        [Patent reference 5] JPA 2002-221866        [Patent reference 6] JPA 2000-330402        [Patent reference 7] JPA 2001-042678        [Patent reference 8] JPA 2000-235320        [Patent reference 9] JPA 11-002984        [Patent reference 9] JPA 2001-345169        [Patent reference 10] JPA 2002-221866        [Patent reference 11] JPA 2004-086090        [Patent reference 12] JPA 2004-252348        [Patent reference 13] JPA 2000-220632        [Patent reference 14] JPA 63-262671        